from storage engine

Federated may crash a server, return wrong result set, return
"ERROR 1030 (HY000): Got error 1430 from storage engine" message
when local (engine=federated) table has a key against nullable
column.

The problem was wrong implementation of function that creates
WHERE clause for remote query from key.


mysql-test/r/federated.result:
  A test case for BUG#33946.
mysql-test/t/federated.test:
  A test case for BUG#33946.
sql/ha_federated.cc:
  Fixed that federated adds " IS NULL " condition to a remote query,
  whereas "IS NOT NULL" requested by original query.
  
  Fixed that federated didn't check for end of key buffer, didn't
  setup key buffer pointer and remaining lenght of key buffer,
  didn't add " AND " between conditions in case original query
  has IS [NOT] NULL condition against nullable column.
  
  Fixed that federated wrongly shifts key buffer pointer by extra
  one byte when key part may be null (was: store_length + 1,
  now: store_length).